What is _("standard output") ? A reserved function?

Question

I'm trying to read linux commands C sources. I started with cat which is one of the shortest, and supposedly easiest, and I found this on line 645:

if ( fstat(STDOUT_FILENO, &stat_buf) < 0)
    die(EXIT_FAILURE, err_no, _("standard output"));

I understand we're handling errors here. The puzzling part is _("standard output") Is it a function called _ ? I could't find its declaration, if it is.

Answer 1

IIRC, _ is some C macro defined somewhere which expands to calling gettext(3)